The present invention relates to hydraulic control valves, and more particularly to high speed, high flow solenoid operated hydraulic control valves.
An electrically controlled variable damping suspension system includes an electrically controlled hydraulic valve which acts as a variable flow restriction from one side to the other of a hydraulic damper, such as a shock absorber. According to the present invention, a solenoid operated hydraulic control valve which is suitable for operating with a pulse width modulated (PWM) control signal can be used. The flow restricting valve is designed to react fast enough to respond to the changing duty cycle of the PWM control signal at the pulse repetition frequency of the PWM control signal. For suspension control purposes, a response time on the order of 10 milliseconds is necessary to secure adequate control. The hydraulic valving system for such a suspension system must operate with flows between zero and 20 gallons per minute (GPM) and with valve pressure drops between zero and 3000 pounds per square inch (PSI).
There are significant obstacles to securing a solenoid operated hydraulic control valve which provides rapid response with small input drive signals in hydraulic systems. In the present invention, a spool valve configuration is used for regulating flow because the areas of the spool lands allow for a force balance, thereby requiring less solenoid force to move the spool.
In hydraulic systems that operate in a high pressure environment with high flow, the moving element of valves can experience great resistance to opening or closing. This is because of the pressure drop across the valve, which creates a differential force due to fluid flow on the spool in a direction to close the valve. This differential force or flow force is caused by the difference in pressure drop on the supply and delivery side of the valve lands as the spool shifts to the opened or closed position. A solenoid operator for such a flow restricting valve must develop sufficient force to overcome any flow force and return spring force during activation of the valve in order to properly shift the spool.
As part of the present invention, an electromagnetic system is provided to produce the required forces at the necessary speeds. A high power solenoid operator generally requires a large armature, having significant mass, and a large solenoid coil, having high inductance, in order to develop significant force with the limited electrical potential and current from the output of a conventional solenoid controller system. Large armature mass and high solenoid coil inductance both cause slow solenoid operator response.
Rather than increasing armature mass and solenoid inductance, according to the present invention the efficiency of a solenoid operator is improved to increase developed force. This efficiency is achieved by reducing the spacing of the non-working gap of the magnetic circuit between the armature and stator, as well as increasing the cross-sectional area of the working and nonworking gaps. In general, the length of the working gap is determined by the operator displacement, and the length of the non-working gap is limited by the necessary clearance between the armature and stator to prevent binding between them during operation. Nevertheless, in accordance with the present invention, the cross-sectional areas of these gaps are increased to diminish losses in the magnetic circuit. This is accomplished without significantly increasing the armature mass or reducing the speed of the solenoid operator response.
Another way to reduce solenoid response time is to reduce the surface tension that occurs between the armature and the stator pole piece or the armature and the cover. When surface tension is high, the armature sticks longer than it should, thereby increasing response time. To alleviate this problem, the present invention includes contouring the surfaces of the armature, stator pole piece, and cover. Not only do these contours reduce surface tension, they also have increased fluid flow in the solenoid and have reduced armature mass. These improvements, including those previously discussed, have produced a lightweight highly responsive solenoid operated hydraulic control valve.